The present invention relates generally to a phased array antenna system. More specifically, the present invention relates to a method for signal handoff between individual antennas of a phased array antenna system.
Electronic communication systems generally employ antennas to both transmit and receive signals. Historically, antennas were mechanically moved to provide coverage of a wide area for transmitting or receiving signals. Phased array antennas are now favored in communication systems for their ability to electronically point the antenna quickly in many different directions without having to mechanically reposition the antenna. Multiple phased array antennas with different orientations may be used for wider coverage than one antenna alone can provide. Signals from the phased array antennas are provided to a communications receiver. Where only a single communications receiver is employed with multiple phased array antennas, a means of electronically switching or handing-off the signal between the multiple phased array antennas and the receiver must be provided.
Phased array transmit/receive antennas operate effectively over a fairly wide range of steering angles encompassed by a solid angle of up to approximately 120xc2x0. In some applications, particularly where the antennas are mounted on a moving platform such as an aircraft, the receiving antennas are moving relative to the signal source. The signal source may be a satellite, another aircraft, or a ground based transmitting/receiving station. A crude method of switching between an active and an inactive phased array antenna would be to simply switch antennas. To minimize power requirements, typically only one antenna would be active or powered on at a given time. As the active antenna beam is steered toward its extreme angle, a second or adjacent antenna would be electronically prepositioned to receive the incoming signal. The second (or inactive) antenna, which is powered off at the time, would be prepared to track the target signal within its coverage region immediately after being powered on and the original or active antenna subsequently being powered off.
At the point of the handoff between the two antennas, a xe2x80x9cmake before breakxe2x80x9d sequence would normally take place as follows: 1) the inactive antenna is powered on, 2) a high speed switch connects the inactive antenna and disconnects the active antenna, then 3) the active antenna is powered off. This crude method results in data loss when the handed-to (second) antenna signal is not xe2x80x9cin-phasexe2x80x9d with the handing-off antenna signal, causing the receiver to temporarily lose phase lock on the switched antenna signal.
Prior to employment of a phased array antenna system, the antenna spacing, frequency of the carrier signal(s), and the desired beam angle (or multiples thereof) for handing-off the signals must be chosen. Given these selections, a threshold phase angle shift or difference of about 15xc2x0 or less between antenna signals would normally be assumed within which phase lock could be maintained. Signals varying in phase angle less than the threshold value are within a negligible phase error region, i.e., a negligible phase angle shift region.
When switching between antennas in communication systems with multiple phased array antennas which share a common receiver a mismatch of signal phase or signal amplitude between the antennas may cause the receiver phase lock loop (PLL) to lose phase lock. Antenna handoff occurring at this mismatch period produces a phase discontinuity in the signal, thereby causing at least a temporary loss of phase lock. Information contained in the antenna signal is lost until the receiver can re-establish phase lock. To minimize the possibility of phase lock loss, the more closely the amplitude and phase are matched prior. to switching antennas, the greater the chance of maintaining receiver lock when switching takes place. A problem in existing systems is that the two antenna beams cannot maintain closely matched beam steering synchronization and delay (time of flight) compensation prior to switching to prevent phase lock loss. A method is therefore required to prevent significant phase differences and subsequent data loss of the signal when preparing for and initiating handoff.
One current approach for preventing data loss (from phase lock loss) during antenna handoff is to use multiple receivers and perform the handoff on demodulated signal data at the output of the receivers. At least two communication receivers are required for this approach, plus additional electronics to save and correlate the history of the demodulated data output from each receiver. This approach has high dollar, weight, and power costs due to the additional communication receiver(s). A method of providing antenna signal handoff is therefore required which reduces the amount and therefore cost of the equipment required to provide this signal handoff.
At the point of antenna handoff, the signal from the handing-off (active) antenna is disconnected, and the signal from the handed-to (inactive) antenna must be connected. The old and new signal will have different amplitudes and phase angles due to differences in beam path, beam angle, and antenna properties as noted above. The switch action itself will also add a small interruption or transient. If this interruption or discontinuity is small (i.e., small enough not to break phase lock loop in the receiver), the risk of data loss will be small. A switch is therefore selected from available switch designs having a speed coincident with the receiver requirements, i.e., the switch speed must not cause loss of phase lock. A receiver phase lock loop (PLL) is normally more susceptible to phase changes of the received signal than to signal perturbations from switch interruption. A method to provide signal handoff between antennas is therefore required wherein the switch action itself is performed at a high enough speed to reduce the potential for signal loss.
The communications receiver design is also an important factor in accomplishing antenna handoff with little or no loss of data. The receiver design normally accommodates the three sources of signal perturbation (amplitude change, phase change and switching transients) with minimal or no data dropout. The integrated system including the antenna and the receiver should therefore provide appropriate response to signal disturbance by not losing signal lock, by recovering from loss of signal lock with no dropping of data, or by recovering in such a manner that the resulting loss of data is corrected by the particular forward error correction scheme utilized. A method of handling phased array antenna handoff is therefore required to minimize impact on the receiver design and meet the criteria of not losing signal lock.
The above drawbacks and problems are addressed by the method of handing-off phased array antenna signals of the present invention. According to the invention, there is provided a phased array communication system which receives and demodulates communication signals using multiple phased array antennas with one receiver. Communication signals are received by the phased array receiving antennas, which are controlled electronically by an antenna controller. The antenna signals go through an antenna select switch matrix in the antenna controller which distinguishes between selected signals from an active handing-off antenna and an inactive handed-to antenna.
Within the antenna controller, the two antenna signals (the handing-off and the handed-to) are routed to a phase comparator which compares the two signals and produces a phase error signal corresponding to the phase difference between the two signals. When a hand-off of signals is desired, a fast switch is enabled, a hand-off control signal from the antenna controller must indicate that antenna hand-off shall take place which requires that the phase error signal must equate to or be less than the predetermined system negligible phase error. The fast switch then performs the handoff switch action. Because the handoff occurs when there is negligible phase difference between the two antennas, phase lock on the switched signal is retained, thereby resulting in no loss or minimal loss of data from the signal handoff.
In one preferred version of the invention, a method for handing-off signals between pairs of phased array antennas is provided, comprising the steps of providing at least two phased array antennas, each designated as one of a handing-off antenna and a handed-to antenna; connecting the phased array antennas to a receiver; communicating a handing-off antenna received signal to the receiver; comparing the handing-off antenna received signal to a handed-to antenna received signal to identify a phase angle shift between the compared signals; predetermining a negligible phase angle shift; handing off the handed-to antenna signal to the receiver when the phase angle shift equals the negligible phase angle shift.
In another version of the present invention, a method for handing-off signals between pairs of phased array antennas while retaining phase lock is provided, comprising the steps of providing at least two phased array antennas, each designated as one of a handing-off antenna and a handed-to antenna; connecting the phased array antennas to a receiver; communicating a handing-off antenna received signal having a first phase angle to said receiver; receiving a handed-to antenna received signal having a second phase angle; comparing both antenna received signals in a phase comparator; producing a phase error signal proportional to a difference between said first and second phase angles; comparing the phase error signal in a control logic circuit to a predetermined negligible phase angle; and handing-off the handed-to received signal to said receiver when said phase error equals said negligible phase angle.
In yet another version of the invention a phased array antenna system is provided, comprising at least two phased array antennas, each designated as one of a handing-off antenna, and a handed-to antenna; a received signal of the handing-off antenna having a first phase angle; a received signal of the handed-to antenna having a second phase angle; a phase comparator to compare the first to the second phase angles; a phase error signal produced by the phase comparator, said signal proportional to the difference between the compared phase angles; an antenna controller; a receiver initially receiving the handing-off antenna received signal; a switch connectably joining both antennas to the receiver; and said switch electrically communicating between each antenna through an antenna select switch matrix, said matrix provided within said controller; wherein the antenna controller compares the phase error signal to a predetermined negligible phase error and when the phase error signal equals the negligible phase error signal the received signal of the handed-to antenna is switched to the receiver.